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50 The radio spectrum has gradually emerged as the preferred channel. Tesla, who built a radio transmitter in Colorado in 1899, believed that his signals had reached Mars. He predicted that interplanetary communica- tion would become the dominating idea of the century that had just begun.? Radar systems developed during World War II gave us a new means for transmitting powerful signals over interplanetary distances. The US Army Signal Corps “bounced” radar signals off the Moon in 1946. Twelve years later, scientists at the Massachusetts Institute of Technology sent a signal to Venus and received the echo. The California Institute of Technology achieved similar results with signals sent to Mars in 1963.4 Military and astronomical radars remain the Earth’s most powerful emitters of electro- anny Ss ene Ba Plaques and Records Rocket pioneer Robert Goddard suggested another way of sending mes- sages in 1920: Interplanetary spacecraft might bear metal plates inscribed with geometrical shapes and astronomical objects. His pro- posal became reality five decades later with the launch of Pioneers 10 and //, the first human-made machines destined to leave our solar system. Those robotic craft carried plaques designed to tell aliens who might find them about the nature of our species and our location in the galaxy. The later Voyagers J and 2 carried “records” containing greet- ings from Humankind. These plaques are destined to be the longest-lived works of our species, Sagan and Drake declared. They will survive virtually unchanged for hundreds of millions, perhaps billions, of years. They are like messages in bottles, cast into the sea of space in the hope that some beachcomber of the stars will find them—and note our existence.” Such small machines are unlikely to be found in the vastness between suns. Sending these messages was more symbolic than practical; we are likely to be the only recipients. The most famous of our deliberate interstellar transmissions was a radio signal sent in 1974 from the largest radio telescope in the world, at Arecibo in Puerto Rico. This was the strongest man-made signal ever transmitted; on its wavelength, the signal would have made our Sun appear to be by far the brightest star of the Milky Way.° The Arecibo transmission was a shout into the cosmic dark, demonstrat- ing that humans want to be noticed—or at least to leave some evidence of their existence. As astronomer Donald Goldsmith put it, we seem to enjoy Sending Our Own Signals magnetic signals. The Arecibo Blast